The invention is in the field of metal-oxide-semiconductor (MOS) field-effect devices, and relates specifically to lateral MOS field-effect transistors of the type having a semi-insulating field plate over the drift region.
Such transistors are well-known in the art, and typical MOS field-effect transistors with a semi-insulating (i.e. very high resistivity) field plate over the drift region are shown in U.S. Pat. No. 4,270,137. Various other prior-art field-plate configurations are shown in U.S. Pat. Nos. 4,288,801; 4,495,513; 4,586,064; and 4,590,509. As noted in U.S. Pat. No. 4,270,137, field plates are incorporated into MOS field-effect transistors in order to achieve a high drain breakdown voltage while maintaining a relatively low steady-state ON resistance.
However, there is also a drawback associated with the use of a semi-insulating field-plate configuration as shown in the prior art devices. In the OFF state, the drift region is depleted, and a potential difference exists across the field plate. When the device is turned ON, the charge accumulated in the semi-insulating field plate cannot quickly dissipate, due to its high resistance, and the drift region surface is in a deep depletion condition, which collapses slowly due to either thermally-generated holes in the depletion region and/or discharge of the field plate. Since the discharge time for typical semi-insulating field plates is long, as is the time to thermally generate holes, the result is a high initial ON resistance which only slowly reaches its desirable low steady-state value. Typically, it will take on the order of several milliseconds or more to reach the low steady-state ON resistance value, thus severely limiting the practicality of this class of device for switching applications. A more detailed analysis of the steady-state and transient switching characteristics of field-plate devices is presented in THE EFFECTS OF SIPOS PASSIVATION ON DC AND SWITCHING PERFORMANCE OF HIGH VOLTAGE MOS TRANSISTORS, Mukhurjee et al, proceedings of the IEDM, December, 1986, pp. 646-649.
Accordingly, it would be desirable to overcome this transient ON resistance problem in semi-insulating field-plate devices, so that these devices, with their advantageous ON resistance and breakdown voltage characteristics, can be used in switching applications.